Borylcyclopentadienides

The cyclopentadienylboranes C 5 H 5 BR 2 1 − 4 (R = NMe 2, 1 / 2 (OCMe 2 ) 2, Pr i, Me) are synthesized from alkali metal cyclopentadienides MCp (M = Li, Na) and BCl(NMe 2 ) 2, ClB (OCMe 2 ) 2, BClPr i 2, and BBrMe 2, respectively. Compound 1 reacts with lithium dialkylamides (LiNMe 2, LDA, LiTMP) in THF or with elemental sodium to give the salts M[C 5 H 5 B(NMe 2 ) 2 ] [M( 5 ) with M = Li, Na]. Compounds 2 and 3 are metalated by LiTMP, and compounds 3 and 4 by cyclopentadienides MCp (M = Li, Na) to give borylcyclopentadienides or boratafulvenes M[C 5 H 5 BR 2 ] M( 6 )−M( 8 ) (R = 1 / 2 (OCMe 2 ) 2, Pr i, Me). Estimated (NMR) room temperature equilibrium constants for the systems 1 − 4 /LiCp in THF are 0.5, 4, >300, and >300, respectively; they show that dialkylboryl substituents stabilize cyclopentadienide ions markedly. The triclinic THF solvate Na(THF)( 5 ) (≡ 10 ) forms an approximately trigonal helix with alternating Na(THF) + and anion units, while the tetragonal tetrahydropyran solvate Na(THP)( 5 ) (≡ 11 ) forms a tetragonal helix. The monoclinic solvate Li(12-crown-4)( 6 ) (≡ 12 ) is molecular, and the orthorhombic Na(THP) 2 ( 8 ) (≡ 13 ) displays a chain structure. The B−C1 distances [154.5(2) for 11, 148.8(8) for 12, and 150.8(4) pm for 13 ], lengthened C1−C2/C1−C5 distances (av) (142.5 for 11, 142.1 for 12, and 143.6 pm for 13 ), slightly lengthened C3−C4 distances (140.0 for 11, 141.1 for 12, and 143.5 pm for 13 ), and shortened C2−C3/C4−C5 distances (av) (140.3 for 11, 137.3 for 12, and 137.9 pm for 13 ) are caused by π interactions ( 11 < 12, 13 ) between the C 5 ring and the boryl function. These interactions are weak in the case of the B(NMe 2 ) 2 group and quite marked for the B(OCMe 2 ) 2 and BMe 2 groups.